1. Field of the Invention
The present invention relates to a back light unit, and more particularly, to a diffuser of a back light unit.
2. Description of the Prior Art
As manufacturing costs decrease and quality improves, liquid crystal displays are widely applied to different kinds of products such as notebooks, personal digital assistants (PDAs), mobile phones, clocks, and so on. Since the liquid crystal displays are passive luminous devices, a back light unit is required for the LCD devices to meet the requirements of night vision and full color imagery.
Basically, the back light unit, which is disposed under the display panel, comprises a luminous means and a diffuser for generating light beams and scattering the light beams uniformly to the display panel so as to produce image on the display panel in advance. According to the position of the luminous means, the back light units are divided into several types, such as the vertical back light unit, in which the light source is generated under the display panel, or the edge type back light unit, in which the light source is projected from the edges. Among all kinds of back light units, since the vertical back light unit generates light beams under the display panel, therefore, the vertical back light units are widely used in large size display panels or those display panels which need high brightness, such as TV sets.
Please refer to FIG. 1 of a schematic diagram of a conventional vertical back light unit 10. The back light unit 10 is disposed under a display panel 18 for providing a light source to the display panel 18. The back light unit comprises a light generator 12 for producing light beams, and a reflective layer 14, which is often composed of a metal layer, for reflecting the light beams generated from the light generator 12 upward to increase the luminous intensity. The back light unit further comprises a diffuser 16 disposed above the light generator 12 for scattering the light beams so that the uniformity of the luminous intensity is improved when the light beams are provided to the display panel 18.
Please refer to FIG. 2 of a top view of the light generator 12. Basically, the light generator 12 is composed by one or a plurality of light tubes 13. In order to increase efficiency, the light tubes 13 usually occupy only parts of the region of the light generator 12 and leave a lot of unoccupied space. Therefore, the light beams generated by the light generator 12 are scattered in various directions. The regions closer to the light tubes 13 have higher luminous intensities. Although the light beams are scattering through the diffuser 16 disposed on the light generator 12 so that the differences among the luminous intensity of each region are reduced, some specific regions, which are too close to the light tubes 13 such as the regions located on the tubes 13 directly, still have relative higher luminous intensities. Thus, when the display panel is used, a highlight region is shown on the specific regions of the display panel 18, which corresponds to the shapes of the tubes 13. The display performance of the display panel 18 is deteriorated.
In conventional back light unit technologies, some methods are used for solving the aforementioned problem. For example, an additional light guide is positioned between the diffuser 16 and the light generator 12 or a light enhancing film is disposed between the diffuser 16 and the display panel 18 for reducing the difference of the luminous intensities in advance. Those methods work successfully when the back light unit 10 outputs light beams with a high luminous intensity. However, when the back light unit 10 outputs light beams with a low luminous intensity or when the screen is almost dark, an obvious highlight region, which has a shape similar to the shape of the light tubes 13, is always shown on the display panel 18 and therefore the display performance is deteriorated.